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Mold can be a problem in areas of your landscaping that are shaded or constantly damp. The mold can kill vegetation, create allergy issues and thrive on the lava rock. There are several ways to kill mold not only to save your plants, but also help improve the overall look of the area. You can use chemical fungicides to attack the mold or a non-toxic mixture made from household products.
Trim any limbs, tree branches and tall shrubs shading the landscape area. A lack of light gives mold the perfect place to grow. Letting in more light will retard mold's growth and actually help kill it. Use a tree trimmer to clip or saw through high branches and loppers for smaller, low-lying branches.
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Spray moldy lava rocks with a fungicide, which won't hurt your vegetation. Spray thoroughly and let the rocks dry. Wait to spray when rain is not forecast. The product needs to work for a few days before it gets washed off. If you prefer not using chemicals, mix baking soda and water in a garden sprayer and spray the rocks evenly. When the rocks are dry, proceed to the next step.
Shovel the lava rocks out of the landscaping and put them aside. Spread more topsoil on the area to raise up low spots that might be collecting water. This is a preventative for future mold growth.
Shovel the lava rocks back in place around the landscaping.